1. Field of the Invention
The subject invention relates to carriers for holding and transporting silicon wafers or the like in the production of semiconductor devices.
2. Prior Art
The mass-production of semiconductor devices involves a series of steps by means of which an electrical circuit in the form of a thin layer of conductive material is applied to the surface of microns-thick wafers of silicon or other suitable materials. During the fabrication process, batches of wafers are transported between processing stations, and processed, in open baskets or carriers commonly referred to as xe2x80x9cboats.xe2x80x9d Conventionally, these carriers are sized to hold 25, 35, or 50 wafers. In some instances, the carriers are loaded and unloaded manually. More commonly, however, the wafers are inserted and removed by automatic mass transfer devices employing robotic arms and multi-pin elevator systems for manipulating some or all of the wafers in a carrier in a single operation or sequence of steps.
For most processing operations, the carriers may be made of Teflon(copyright), polypropylene, various metals, polysilicon, silicon carbide, or other convenient materials. Some of the processing steps are carried out at very high temperatures, however, and to withstand these temperatures and avoid contamination, the carriers used are commonly made of nearly pure quartz.
Typically, a prior art quartz carrier comprises a plurality of parallel quartz rods having a series of transverse grooves or slots spaced lengthwise. Quartz cross members welded to the ends of the rods define a rigid, open basket adapted to receive and support a batch of wafers upstanding edgewise in spaced, parallel relation. FIG. 2 illustrates one such prior art carrier.
To facilitate the creation and implementation of industry-wide manufacturing practices and quality control procedures, industrial standards have been adopted regulating the configuration, construction, and specifications of these carriers. Pursuant to those standards, in a carrier adapted to retain 25 wafers, the grooves in the supporting rods are to be positioned 0.1875 inches, plus or minus 0.002 inches, on-center. For 35 wafers, the grooves are 0.1475 inch, plus or minus 0.002 inch, on-center, and for 50 wafers, 0.0937 inch, plus or minus 0.002 inch on-center.
For certain of the steps in the production process requiring exposure of the wafers to high temperatures, it is conventional to load a number of carriers with their batched wafers onto a supporting structure of some sort by means of which the wafer-bearing carriers are transported through the heat-treating chamber of a diffusion furnace. In one prior art system, the carriers are mounted to an elongated sled or wheeled cart and pass through the heating chamber in a single file. This arrangement proved undesirable for a number of reasons. Primary among these is that the wafers in the train of carriers are processed sequentially and are thus subject to variations in temperature and ambient operating conditions.
A preferred form of transport system and the one presently in common use is illustrated in FIG. 1. As shown, the supporting transport structure is an elongated xe2x80x9cpaddle,xe2x80x9d commonly fabricated of silicon carbide or another suitable material, cantilevered from a motorized track. In this arrangement, the carriers pass through the hot region of the furnace in a transverse line perpendicular to the direction of motion. Advantageously, the wafers in all of the carriers are processed at the same time, at the same temperature, and under substantially the same ambient conditions. Additionally, since the wafers are moving through the furnace edgewise, their passage through the chamber has little eddy-generating affect on the chamber atmosphere. The relative air flows smoothly through the spaces between adjacent wafers and across (i.e., parallel to), rather than against the wafers"" surfaces.
By virtue of these advantages, the use of this system imposes several requirements on the construction, configuration, and operation of the wafer carriers. One of these is to maximize the number of wafers that can be loaded into the allotted space on the paddle. Another is to provide manual means for conveniently and safely loading, unloading, and manipulating individual carriers without damaging or disturbing their wafer cargo.
Addressing the first of these requirements, as will be apparent, the cross members at the ends of the prior art carriers exemplified in FIG. 2 occupy a substantial amount of furnace space that might otherwise be utilized for processing additional wafers. U.S. Pat. No. 4,515,104 represents an attempt to minimize the space taken up by the rod-supporting cross members and thereby maximize the capacity carriers capacity. Its approach is to space the cross members inwardly of the ends of the rods. While in principle this arrangement allows adjacent carriers to be positioned in contiguous end-to-end relationship, carriers embodying the construction suffer from certain serious deficiencies. For one, the end-most wafers, lacking frontal protection, are subject to being displaced or jarred out of position and to contacting one another. For another, the cross members, being positioned within the wafer-containing portion of the carrier, prevent or severely limit the use of most mass transfer devices for the automated loading and unloading of wafers. For still another, the cross members deflect the flow of gases within the carrier and create vortices and eddies in the spaces between the adjacent wafers that subject the wafers to uneven heating and exposure.
Clearly, there is a need for, and one of the objectives of the subject invention is the provision of a carrier that affords the maximum wafer-loading capacity without the deficiencies inherent in the known prior art carriers.
With respect to the second requirement, a number of devices have been designed for manually handling quartz carriers. Generally, these fall into two categories: One of these employs receptacles mounted to the carrier, either axially or transversely, and a split or forked handle having hooks or tines adapted for insertion into the receptacles. Examples of these devices are shown and described in U.S. Pat. Nos. 3,861,733, and 4,515,104. Suffice to say these devices suffer from a number of widely recognized inherent deficiencies, such as, fragility, difficulty of alignment, instability, susceptibility to slippage and uncontrolled release, and in the case of the axially-mounted handle, lack of random accessibility to the carriers, that render them less than satisfactory.
The second category of manipulating devices employs a tool assembly adapted to serve two functions, the first, supporting the weight of the carrier, and the second, limiting the rotational motion of the carrier about the weight-supporting means. These devices are illustrated by U.S. Pat. Nos. 4,572,101, and 5,033,406. Assemblies of this type are generally side mounted and have the principal advantage of affording the user random access to individual carriers in a string. The price of the advantage, however, is the substantial cost, the weight penalty, and the increased fragility associated with the auxiliary mounting structures required for attachment of the tool to the carrier. A viable alternative is needed, and accordingly, another object of the subject invention is the provision of a side-mounted tool assembly for manipulating the carrier safely and efficiently that avoids the deficiencies of the known prior art assemblies.
The subject invention utilizes a pair of thin, substantially flat quartz end pieces in place of the conventional xe2x85x9cxe2x80x3 or ⅝xe2x80x3 rod or tubular cross members employed in the prior art carriers to join the wafer supporting rods and stiffen the carrier structure. Not more than xe2x85x9 inch thick, these end pieces afford a saving of almost an inch in the overall length of a carrier, providing space for as many as two additional carriers with their wafers in the typical furnace hot zone. This represents a potential increase of 30% in the yield at each stage of processing.
In addition to enhanced yield, the use of thin, flat end pieces affords the subject invention a second advantage in eliminating the need for excess structural components. Whereas all of the prior art side mounted handle assemblies require auxiliary support and motion limiting apparatus on the carrier or on the handle, or both, the subject invention employs only one support member and utilizes the end pieces to limit the rotation of the carrier.
A further advantage derived from the employment of the flat end pieces to support the axial rods is that a carrier made in accordance with the invention is fully adapted for use with all of the known automated-transfer systems presently in the field.
With this brief overview as background, other objects, features, advantages, and applications of the invention will become apparent from the following detailed description of the construction and operation of what is presently considered to be its preferred embodiment as illustrated in the accompanying drawings, wherein: